This invention relates to an apparatus and a method for assembling a cathode-ray tube bulb assembly and mount assembly, and particularly, to a method of aligning a multibeam electron gun assembly in a cathode-ray tube bulb.
In a commercial cathode-ray tube such as a color television picture tube of the apertured mask type having a three-color viewing screen structure, the viewing screen structure is photographically printed using light centers simulative of the position of the deflection center of each of the three electron beams in the final tube. A mount assembly comprising a three beam electron gun is subsequently installed in the tube. During the assembly of the electron gun structure in the tube bulb, the axis of each electron beam path must be oriented to coincide with the light centers used to print the viewing screen structure within a desired rotational tolerance about the central longitudinal axis of the tube. In commercial color television picture tubes using dynamic convergence circuitry, a mount assembly, including an electron gun assembly having three cathodes in fixed orientation, ordinarily must be positioned in the tube within three degrees of rotation. In a commercial color television picture tube using no dynamic convergence circuitry or simplified dynamic convergence circuitry, a more accurate rotational positioning of the mount assembly to about one half degree is usually required.
In one prior method for assembling a multi-beam electron gun structure, the alignment is accomplished by several separate assembly operations. During the mount assembly operation, the central longitudinal axis of the electron gun assembly is aligned with the stem axis and the cathode axes are rotationally aligned with the stem leads. Then, the electron gun assembly is attached to the stem leads with metal wires and ribbons to form a mount assembly. In the subsequent mount sealing operation, the preassembled mount assembly is positioned and oriented with respect to the bulb assembly and then sealed to the bulb assembly on a sealing unit. In this mount sealing operation, the mount assembly is held rotationally with the stem leads positioned within aligned holes on the sealing machine. Since the holes include a clearance for loading and the mount assembly includes assembly tolerances, the rotational alignment of the mount assembly with respect to the screen structure can only be maintained within about three degrees of rotation. In addition, since the mount assembly is preassembled and transported to the sealing machine, the fragile wires supporting the electron gun assembly may be accidentally bent thereby misaligning the electron gun assembly with the stem leads. This may result in an angular misalignment of the electron gun assembly when the stem leads are used to angularly align the mount assembly with the bulb assembly.
In another prior method for assembling a multibeam electron gun structure, as described in U.S. Pat. No. 3,807,006 issued to Segro et al., the alignment is accomplished by mechanically sensing the position of the electron gun assembly with respect to the bulb assembly. While this method is an improvement in that it obviates the necessity to align the electron gun assembly with the stem axis which is in turn aligned with respect to the bulb assembly, this method entails the necessity of physically contacting the electron gun assembly thereby introducing its own errors into the total alignment error.
In yet another method of assembling a cathode ray tube having a bulb assembly and a mount assembly, described in U.S. Pat. No. 3,962,764 issued to Stewart et al. on June 15, 1976, the bulb assembly first is positioned in a predetermined orientation. Next, the mount assembly, which includes a multibeam electron gun assembly, is positioned in a location spaced from the bulb assembly with the central longitudinal axis of the mount assembly coincident with the central longitudinal axis of the bulb assembly. Next, the rotational position of the electron gun assembly about the coincident longitudinal axis is optically sensed with respect to the positioned bulb assembly by use of split images. The mount assembly is then rotated about the coincident longitudinal axis until the split images are aligned thus indicating that the electron gun assembly is at a prescribed rotational orientation with respect to the bulb assembly. Then, while maintaining this rotational orientation, the mount assembly is moved along the longitudinal axis to a desired longitudinal location with respect to a faceplate panel of the bulb assembly at which time the mount assembly is then permanently fixed to the bulb assembly.